In the last 50 years, the “green revolution” has contributed to higher yields, greater stability of harvests and, as a result, higher farm income. This “green revolution” (Borlaug, 2000) resulted in part from conventional plant breeding yielding superior plant varieties, but also from (1) expansion of land mass for cultivation; (2) improved agricultural practices through fertilizer application and crop rotation and (3) pesticide and herbicide use which decrease the loss in productivity arising from plant pathogens and insect pests. However, the limits of these strategies are currently being realized. One approach, which will aid in addressing these limitations, is the development of crop species that have increased nitrogen use efficiency, thus increasing productivity on land of both marginal and good quality.
Nitrogen is an essential, limiting nutrient required for plant growth (Vance, 1997). To increase plant productivity, high levels of N-fertilizer are normally applied to crops. Global usage of fertilizer has increased from 32 to 87 million MT (1970 to 2000) and is projected to increase to 240 million MT by 2050 (Tilman, 1999). In Canada and the US alone 25 million tons of N, P, and K fertilizer are applied annually at a cost of $10 B (US) (Tisdale et al., 1985). To sustain high crop yields, up to 200 kg per hectare of fertilizer is often applied to many agricultural species. For example canola shows a positive correlation for seed yield, with fertilizer applications of up to 135 kg per hectare, after which there is a slight decrease in yield (Porter 1993). In addition to the economic costs of fertilizer application, there are environmental effects including the deterioration of soil quality, pollution and health hazards. The extensive use of NH4+, urea or nitrate as fertilizers all have negative impact on the environment. Heavy use of urea and ammonium based fertilizers causes acidification of soils, which results in decreases in both yield and the quality of crops (Goulding et al., 1998). Nitrate based fertilizers are problematic in that they do not remain in the soil, with up to 50% being lost through leaching (Carpenter et al., 1995). The release of gaseous nitrogen compounds from fertilizers also exacerbates acid rain (Matson et al., 1998) and the greenhouse effect (Tilman, 1999). Thus there is a need for plants that are capable of utilizing nitrogen more efficiently so that less nitrogen is required for the same yield or higher yields may be obtained with current levels of nitrogen use.